In an ink jet printer, of charge controlled type, for example, in which ink under pressure is projected from a nozzle, ink is pumped by a pump from an ink vessel to be supplied to an ink projecting head. The ink projecting head is provided with a vibrator, which vibrates at a given frequency to impart pressure oscillations of a given period to the ink received within the head. The ink is normally projected from the nozzle under a given pressure and with a given pressure oscillation, whereby the ink stream is separated into ink particles at a given distance from the nozzle. A charging electrode is disposed forwardly of the nozzle, and when a charging voltage is applied between the charging electrode and the ink contained within the head in synchronism with the separation of the stream of an ink jet into ink particles, the resulting ink particles will bear an electric charge which depends on the magnitude of the charging voltage. The charged ink particles are deflected by an electric field established across a pair of deflection electrodes for impingement upon a record paper. An ink jet printer of the type described is well known, and is disclosed, for example, in U.S. Pat. No. 3,596,275 issued to Richard G. Sweet and U.S. Pat. No. 4,045,770 issued to Robert Walker Arnold et al.
A plugging of the nozzle presents a problem in an ink jet printer of the type described. Specifically, solvent may be evaporated from the ink disposed inside or outside the nozzle over time during which the printer remains out of use, and consequently, the ink may be solidified or has its viscosity increased to a high value, preventing a proper projection of an ink stream. The plugging may also be caused by deposition of dust such as minute fragments of paper into the ink around the nozzle to close it.
When the nozzle is plugged, the ink pressure in the head which projects an ink stream rises during the time an associated pump is being driven. If the pumping operation is allowed to continue and the pressure rises, a deformation or damages of the head or ink feed pipe or a burn-out of the pump may result. To avoid these occurrences, it has been the prior art practice to detect the ink pressure which is supplied to the head and to interrupt the operation of the pump immediately upon a given pressure being reached, producing an alarm to switch the system into a standby mode.
In this respect, it is to be noted that an alarm may be produced and the system may be switched to a standby mode by interrupting the operation of the pump when a phase retrieval or a deflection control failed to establish a proper condition, based on the assumption that such failure is caused by the imperfect projection of an ink stream. In another mode, an indicator which requires a call-up for a serviceman may be energized.
From the foregoing, it will be seen that there has been a relatively frequent occurrence in the prior art practice that a standby mode in which the projection of an ink stream is interrupted is established due to the plugging or the possibility of plugging of the ink projecting head, in particular, when the printer is used infrequently or the printer is left out of use over an increased length of time. Accordingly, there arises a problem that a print-out at a desired time is disabled or a problem that the maintenance required an increased amount of labor or manual intervention in checking or cleaning the head.